This invention relates generally to a computer-controlled method and apparatus for fabricating a three-dimensional (3-D) object and, in particular, to an improved method and apparatus for building a multi-material or multi-color 3-D object directly from a computer-aided design of the object in a layer-by-layer, but not point-by-point fashion. The presently invented method is referred to as a Full-Area Sintering Technique (FAST).
Layer manufacturing (LM) or solid freeform fabrication (SFF) or is a new fabrication technology that builds an object of any complex shape layer by layer or point by point without using a pre-shaped tool such as a die or mold. This process begins with creating a Computer Aided Design (CAD) file to represent the geometry or drawing of a desired object. This CAD file is converted to a proper solid interface format such as the stereo lithography (.STL) format. The geometry file is further sliced into a large number of thin cross-sectional layers with each layer being comprised of coordinate point data. In a commonly used layer-wise data format called Common Layer Interface (CLI), the contours (shape and dimensions) of each layer are defined by a plurality of line segments connected to form polylines on an X-Y plane of a X-Y-Z orthogonal coordinate system. The layer data are converted to tool path data normally in terms of computer numerical control (CNC) codes such as G-codes and M-codes. These codes are then utilized to drive a fabrication tool for defining the desired areas of individual layers and stacking up the object layer by layer along the Z-direction.
The SFF technology makes it possible to convert a CAD image data directly into a three-dimensional (3-D) physical object. The technology has been widely used in applications such as verifying CAD database, evaluating engineering design feasibility, testing part functionality, assessing aesthetics, checking ergonomics of design, aiding in tool and fixture design, creating conceptual models and marketing tools, producing medical or dental models, generating patterns for investment casting, reducing or eliminating engineering changes in production, and providing small production runs.
The SFF techniques may be divided into three categories: layer-additive, layer-subtractive, and hybrid (combined layer-additive and subtractive). A layer additive process involves adding or depositing a material to form predetermined areas of a layer essentially point by point; but a multiplicity of points may be deposited at the same time in some techniques, such as of the multiple-nozzle inkjet-printing type. These predetermined areas together constitute a thin cross-section of a 3-D object as defined by a CAD geometry. Successive layers are then deposited in a predetermined sequence with a layer being affixed to its adjacent layers for forming an integral multi-layer object. A 3-D object, when sliced into a plurality of constituent layers or thin sections, may contain features that are not self-supporting and in need of a support structure during the object-building procedure. These features include isolated islands in a layer and overhangs. In these situations, additional steps of building the support structure, also on a layer-by-layer basis, will be required of a layer-additive technique. An example of a layer-additive technique that normally requires building a support structure is the fused deposition modeling (FDM) process as specified in U.S. Pat. No. 5,121,329; issued on Jun. 9, 1992 to S. S. Crump.
A layer-subtractive process involves feeding a complete solid layer of a material to the surface of a support platform and using a cutting tool (normally a laser) to cut off or somehow degrade the integrity of the un-wanted areas of this solid layer. The solid material in these un-wanted areas of a layer becomes a part of the support structure for subsequent layers. These un-wanted areas are hereinafter referred to as the xe2x80x9cnegative regionxe2x80x9d while the remaining areas that constitute a cross-section of a 3-D object are referred to as the xe2x80x9cpositive regionxe2x80x9d. A second solid layer of material is then fed onto the first layer and bonded thereto. The same cutting tool is then used to cut off or degrade the material in the negative region of this second layer. These procedures are repeated successively until multiple layers are laminated to form a unitary object. After all layers have been completed, the unitary body (or part block) is removed from the platform, and the excess material (in the negative region) is removed to reveal the 3-D object. This xe2x80x9cdecubingxe2x80x9d procedure is known to be tedious and difficult to accomplish without damaging the object. An example of a layer-subtractive technique is the well-known laminated object manufacturing (LOM), disclosed in, for instance, U.S. Pat. No. 4,752,352 (Jun. 21, 1988 to M. Feygin).
A hybrid process involves both layer-additive and subtractive procedures. An example can be found with the Shape Deposition Manufacturing (SDM) process disclosed in U.S. Pat. No. 5,301,863 issued on Apr. 12, 1994 to Prinz and Weiss. Such a process is complicated and difficult to operate. It also requires the operation of heavy and expensive equipment.
Another good example of the layer-additive technique is the 3-D powder printing technique (3D-P) developed at MIT; e.g., U.S. Pat. No. 5,204,055 (April 1993 to Sachs, et al.) and U.S. Pat. No. 6,007,318 (Dec. 28, 1999 to Russell, et al.). This 3-D powder printing technique involves dispensing a layer of loose powders onto a support platform and using an ink jet to spray a computer-defined pattern of liquid binder onto a layer of uniform-composition powder in a point-by-point fashion. The binder serves to bond together the powder particles on those areas (positive region) defined by this pattern. Those powder particles in the un-wanted areas (negative region) remain loose or separated from one another and are removed at the end of the build process. Another layer of powder is spread over the preceding one, and the process is repeated. The xe2x80x9cgreenxe2x80x9d part made up of those bonded powder particles is separated from the loose powders when the process is completed. This procedure is followed by binder removal and impregnation of the green part with a liquid material such as epoxy resin and metal melt. Although several nozzle orifices may be employed to dispense several droplet streams at the same time, this 3D-P process remains to be essentially a point-by-point process, being characterized by a slow build speed.
This same drawback is true of the selected laser sintering (SLS) technique (e.g., U.S. Pat. No. 4,863,538, Sep. 5, 1989 to C. Deckard, U.S. Pat. No. 4,938,816, Jul. 3, 1990 to J. Beaman, et al., and U.S. Pat. No. 5,316,580, May 31, 1994 to Deckard). The SLS technique involves spreading a full-layer of loose powder particles and uses a computer-controlled, high-power laser to partially melt these particles within predetermined areas (positive region) in a point-by-point fashion. Commonly used powders include thermoplastic particles, thermoplastic-coated metal particles, metal-coated ceramic particles, and mixtures of high-melting and low-melting powder materials. These point-wise procedures are repeated for subsequent layers, one layer at a time, according to the CAD data of the sliced-part geometry. The loose powder particles in the negative region of each layer are allowed to stay as part of a support structure. The sintering process does not always fully melt the powder, but allows molten material to bridge between particles. Commercially available systems based on SLS are known to have several drawbacks. One problem is that the need to use a high power laser makes the SLS an expensive technique and un-suitable for use in an office environment. Again, the spot-by-spot or point-by-point laser scanning is a very slow procedure, resulting in a low object-building speed.
In U.S. Pat. No. 5,514,232, issued May 7, 1996, Burns discloses a method and apparatus for automatic fabrication of a 3-D object from individual layers of fabrication material having a predetermined configuration. Each layer of fabrication material with desired shape and dimensions is first deposited on a carrier substrate in a deposition station. The fabrication material along with the substrate are then transferred to a stacker station. At this stacker station the individual layers are stacked together, with successive layers being affixed to each other and the substrate being removed after affixation. Lamination-based LM techniques that require radiation curing of solid sheet polymer materials layer by layer can be found in U.S. Pat. No. 5,174,843 (Dec. 29, 1992 to M. Natter) and U.S. Pat. No. 5,352,310 (Oct. 4, 1994 to M. Natter). Natter""s technique is limited to high-energy radiation-curable polymer materials in a solid sheet form. Disclosed in U.S. Pat. No. 5,183,598 (Feb. 2, 1993 to J-L Helle, et al.) is a process that includes preparing thin sheets of a fiber- or screen-reinforced matrix material. In these composite sheets, the matrix material exhibits the feature that its solubility in a specific solvent can be changed when the material is exposed to a specific radiation. Selected areas of individual sheets are radiated to reduce the solubility. The un-irradiated portion (the negative region) of individual layers remains soluble in the solvent. The stack of sheets are affixed together to form an integral body, which is immersed in the solvent that causes the desired object to appear. This process exhibits the following shortcomings:
(1). A high-power radiation source (e.g., a high-power laser beam) is required. High energy radiation sources and their handling equipment (for reflecting, focusing, etc) are expensive. Furthermore, they are not welcome in an office environment.
(2). When a screen is used as the reinforcement, the screen in the negative region is difficult to get dissolved in the solvent particularly if this screen is made of metal or ceramic materials. A strong acid is needed in dissolving a metal screen.
Lamination-based LM techniques that involve transferring thin sections of powders, prepared by electrophotographic or electrostatic attraction, to a stacking station are disclosed in U.S. Pat. No. 5,088,047 (Feb. 11, 1992 to D. Bynum), U.S. Pat. No. 5,593,531 (Jan. 14, 1997 to S. M. Penn), and U.S. Pat. No. 6,066,285 (May 23, 2000 to Kumar). In Bynum""s process, a drum-shaped electrophotographic element is first prepared. A light image corresponding to a cross-section of an object generated by a computer is projected into this element by line-by-line laser scanning, coordinated with rotational speed of the drum to selectively dissipate the charge thereon, thereby creating an electrostatic latent image on the element. The element, along with the latent image thereon, is then rotatably transferred to a plurality of developer stations, which respectively apply forming powders (toner) to different areas of the electro-photographic element. For each layer, at least two developer stations are needed to apply two different powders to the positive and negative regions, respectively, for building the object cross-section (positive region) and the support structure (negative region). These areas of powders are then electrostatically attracted to a surface of an endless flexible belt, which carries these patterned powders to a fixing station where the powder particles in the positive region are made tacky by the application of heat or solvent vapor. The tackified lamina is then transferred to a stacking station and laid up onto a support platform or a previous layer to form a layer of both the object cross-section and support structure. The above steps are repeated in the same sequence to lay up multiple laminas to form a block of laminas. The powder materials in the negative regions for forming the support structure are usually made of lower melting materials and can be removed by heat from this block at the end of the build process to reveal the desired 3-D object. A fundamentally similar process is disclosed in Penn""s patent and Kumar""s patent. The processes specified in these three patents (U.S. Pat. Nos. 5,088,047, 5,593,531, and 6,066,285) have the following drawbacks:
(1) At least two toner developing stations are required, one for forming the part (object) and the other for the support structure. For every layer of the same object-building material, two different types of powders have to be precisely deposited electrostatically, in sequence and in registration, onto complementary areas of a layer. This is difficult to accomplish without suffering cross-contamination.
(2) It is well-known in the art of electrophotography that most of the conductive particles (e.g., metal powders) do not work well with charging devices. This effectively eliminates the freeform formation of many metallic parts if metal particles are the primary body-building material of the part being built. In contrast, the presently invented method provides an effective way of eliminating this limitation, making our method so much more versatile. In this method, we make use of a simple powder-feeder to supply and evenly spread up a layer of a primary object body-building powder material (e.g., metal), analogous to the powder-feeding step in afore-mentioned SLS and 3D-P processes. We then use electrophotography techniques to form, develop, and transfer toner images of a binder powder (to bond or sinter together the underlying primary body-building powder particles) and a plurality of property modifying powders (modifier powders, e.g., coloring agent), simultaneously or in sequence. The binder and modifier powders collectively occupy only a small fraction of the object cross-section being built.
(3) These three prior art electrophotography methods are limited to loose powders as the starting primary body-building materials. Other forms of material such as a porous substrate (e.g., comprising fiber preform as a reinforcement for a composite) can not be used in these processes.
(4) Penn""s and Kumar""s methods are essentially limited to the fabrication of an object of homogeneous material composition and are not easily or readily adapted for the preparation of a multi-material or multi-color object in which the material composition or color pattern can be varied from point to point. Bynum""s method, in principle, allows for variation of material composition or color pattern from point to point, like in the case of the traditional 2-D printing process that involves developing and transferring multi-color toner images to a sheet of paper. In real practice, however, the electrostatic attraction in a traditional electrophotography system can only handle a thin layer of light-weight toner powder at a time, up to 10 xcexcm or less in thickness. It would take an extremely long time to build up a 3-D model of, say, 100 mm in thickness. In contrast, in our method, the powder feeder can feed layers of heavy- or light-weight powder of which the layer thickness can be varied from very thin to very thick. With the primary body-building powder occupying the majority of the object volume (typically 70% to 95%), the electrophotography device is required to provide only a small amount of binder and modifier powders at a time. Further, in our method, in the negative region of a layer where the primary body-building powder receives no binder, the powder particles serve to provide the needed support structure. It is not necessary to carry out the extra steps of developing a support structure toner image and transferring this image to the negative region of a layer (where the positive region of the layer is already deposited with the image material) in such a fashion that the two complementary regions of different materials must perfectly match (in registry) in shapes and thickness.
Despite these shortcomings of the afore-mentioned three patents, the concept of adapting electrophotography techniques for transferring powder materials in a LM system has proven to be very useful.
Due to the specific solidification mechanisms employed, many LM techniques are limited to producing parts from specific polymers. For instance, Stereo Lithography (SLa) and Solid Ground Curing (SGC) rely on ultraviolet (UV) light induced curing of photo-curable polymers such as acrylate and epoxy resins. The photo-curable polymer in these two cases constitutes the vast majority of the material in the resulting 3-D object. Any other ingredient such as an additive or reinforcement represents at best a minority phase in the structure. The photo-curable polymer in the resulting structure is a xe2x80x9chostxe2x80x9d while any additive, if present, is just a guest. The host provides the basic structural integrity of the 3-D object. Unfortunately, photo-curable polymers alone normally do not have good mechanical strength and toughness.
The above state-of-the-art review has indicated that all prior-art layer manufacturing techniques have serious drawbacks that prevent them from being more widely implemented.
Therefore, an object of the present invention is to provide an improved layer-additive method and apparatus that can be used for producing a multi-material or multi-color 3-D object.
Another object of the present invention is to provide a computer-controlled method and apparatus for producing a part on a layer-by-layer, but not point-by-point basis (hence, with a high build speed).
It is a further object of this invention to provide a computer-controlled object building method that does not require heavy and expensive equipment such as a high-power laser system.
It is another object of this invention to provide a method and apparatus for building a CAD-defined object in which the support structure is readily provided during the layer-adding procedure.
Still another object of this invention is to provide a layer manufacturing technique that places minimal constraint on the range of materials that can be used in the fabrication of a 3-D object. Further, the material composition or color of the object can be varied from spot to spot and/or from layer to layer.
The Method
The objects of the invention are realized by a method and related apparatus for fabricating a three-dimensional, multi-material or multi-color object on a layer-by-layer basis (but not point-by-point) and in accordance with a computer-aided design (CAD) of this object. The object is made from at least a primary body-building powder material, a binder powder, and at least a property-modifying material in fine powder form (hereinafter referred to as modifier powder). This modifier powder can contain a colorant. The design contains data on the geometry (shape and dimensions) and material composition distribution (and/or color pattern). The data preferably is sliced into layer-wise data sets with each set defining the geometry and material composition of a constituent cross-section of the object. Basically, the method includes, in combination, the following steps:
(a) providing a work surface on a support platform that lies substantially parallel to an X-Y plane of an X-Y-Z Cartesian coordinate system defined by three mutually orthogonal X-, Y- and Z-axes;
(b) feeding a first layer of a primary body-building powder material to the work surface (e.g., by using a traditional powder feeder commonly used in selected area sintering and 3-D powder printing processes);
(c) operating an electrophotographic powder deposition means to create transferable powder toner images of a binder powder and at least a modifier powder in accordance with the CAD design; (A plurality of modifier powders may form separate toner images or may be combined to form one composite toner image.)
(d) transferring the transferable modifier and binder powder images, one image at a time, in a desired sequence onto the first layer of the primary body-building powder material;
(e) applying energy means to fuse said binder powder, allowing the resulting fused binder fluid to permeate downward through the first layer of primary body-building material for bonding and consolidating the particles in the first layer to form a first cross-section of the object; (Bonding and consolidating are hereinafter collectively referred to as sintering in the present context.)
(f) feeding a second layer of a primary body-building powder material onto the deposited first layer and repeating the operating, transferring, and applying steps to form a second cross-section of the object; (The material distribution and color pattern in the second cross-section may be different from those of the first cross-section.)
(g) repeating the feeding, operating, transferring, and applying steps to build successive layers of possibly varying material compositions and/or color patterns in a layer-wise fashion in accordance with the CAD design for forming multiple layers of the object; and
(h) removing un-bonded powder particles, causing the 3-D object to appear.
In this instant method, the steps of applying energy means could include pre-heating a layer of primary body-building powder material to a temperature above the melting point of the binder powder. This is done so that the binder powder, when transferred and deposited onto the predetermined areas (positive region) of a corresponding pre-heated body-building material powder layer, will be quickly melted to become a fluid that permeates through the gaps between fine particles of the body-building material powder. This binder fluid, when solidified, will bond and consolidate the powder particles in the positive region, leaving the powder particles in the negative region un-bonded (free from binder). The particles in the negative region stay as part of a support structure. As one can easily see, in this method, any material that can be made into a fine powder form can be used as a primary body-building material and can be easily fed and evenly spread up to form a layer. This is a very significant advantage over other prior art electrophotography-based LM techniques.
The binder powder could include a resin composition that can be cured or hardened with heat, ultra violet light, electron beam, ion beam, plasma, microwave, X-ray, Gamma ray, or a combination thereof. Alternatively, the binder powder could include a lower-melting material that can be readily fused to become a fluid. Once permeating through a layer of primary body-building powder material for providing bridges between particles, the binder fluid can be cooled down to below the melting point of the binder material and be solidified. Preferably, the steps of applying energy means are carried out in such a manner that successive layers are affixed together to form a unitary body of the 3-D object. This can be easily accomplished by allowing the fused binder fluid to have sufficient time to permeate through the current layer of body-building powder material and reaching the top surface of the previously deposited layer.
In the instant invention, the working principle of the electrophotographic powder deposition means can be selected from a range of electrostatic printer or photocopier mechanisms. For instance, electrophotographic powder deposition means can include, but not limited to (1) planar capacitor dot matrix charging device and (2) combined corona discharging/thin photoconductive charge receptor/scanning laser imaging devices. The electrophotographic powder deposition means is characterized by the following features:
(4) It provides a 2-D pattern or xe2x80x9clatent imagexe2x80x9d of electrostatic charges to attract fine powder particles of the binder composition and/or modifiers to form these binder/modifier particles into a toner xe2x80x9cimagexe2x80x9d (thin section of powder particles) in selected areas of a powder layer; these areas being programmable and predetermined by a computer. These areas, corresponding to the positive region of a layer, are defined by the layer data of a CAD design for the object to be built. A full area of the binder powder and/or modifier powder is formed and transferred to deposit onto a layer of body-building powder material, equivalent to a process of xe2x80x9cphoto-printingxe2x80x9d. The binder powder xe2x80x9cphoto-printedxe2x80x9d to the positive region of a body-building powder material layer will help sinter the particles therein, forming a cross-section of the 3-D object. The modifier powder image transferred to the same region of a layer will impart desired physical properties (e.g., color appearance) to this layer. The primary body-building powder particles in other areas of the same layer, not receiving any binder powder composition, will remain as isolated, loose particles that serve as part of a support structure. As opposed to the case of conventional selected laser sintering (SLS) in which a laser beam is used to sinter the powder spot by spot (essentially point by point), the presently invented method builds the part area by area (up to one full layer at a time). This is also in sharp contrast to operating an inkjet printhead to print adhesive onto a layer of powder in a point-by-point fashion in a conventional 3D powder printing (3D-P or MIT) process.
(5) The binder powder, once deposited, is melted in such a manner that the binder fluid flows around to provide a bridge between primary body-building particles in the positive region. The binder can bond together these particles to impart sufficient strength and rigidity to the layer for easy handling and for maintaining the part dimensional accuracy during the formation of subsequent layers. If the binder contains a photo-curable adhesive composition, the pre-heat energy intensity and the energy of the imposing light source (heat and light constituting the energy means) should be provided in such a fashion that successive layers can be affixed together to form a unitary body of the 3-D object.
(6) If the binder contains a heat-fusible material composition, a complete body-building powder layer can be pre-heated by other heat sources (e.g., infrared, IR) disposed near the object-building zone to a temperature (Tpre) sufficient for melting the binder composition. After a selected duration of time, this heat source may be switched off to allow the binder fluid (already permeating through a layer) to solidify. If the layer of primary body-building material is already mixed with component compositions of a binder (excluding a photo-initiator, for instance), the electro-photographic powder deposition means may be used to transfer an image of the photo-initiator powder to the positive region of the layer. The pre-heat temperature Tpre may be so chosen that it is capable of promoting the curing reaction once initiated by the photo-initiator along with an incident light, but insufficient for initiating the curing reaction of the binder compositions by the pre-heat alone. This auxiliary heat would help accelerate the cure reaction and significantly reduce the light intensity requirement that would otherwise be imposed upon the light source. In this favorable situation, the light source can be just based on an ordinary ultraviolet (UV) light source. No expensive high-power laser beam, electron beam, X-ray, Gamma-ray or other high-energy radiation is necessary.
(7) The physical sizes of the binder powder image forming area (electrostatically charged substrate area of a photo-receptor, for instance) of this electrophotographic powder deposition means are preferably sufficient to cover the complete envelop of a primary body-building powder layer so that a complete cross-section of the 3-D object can be built in one binder powder image transfer. This is one of the advantages over the case of conventional selected laser sintering (SLS) which requires aiming a laser beam to one spot at a time (spot being micron- or sub-millimeter-sized). It would take a much longer time for a laser beam to fuse and sinter the particles of a complete cross-section in a spot-by-spot or point-by-point fashion. Further, since binder powder image can be exactly identical to the desired cross-section of a layer, this instant invention also has a significant advantage over the conventional 3D-P process, which involves ejecting adhesive droplets essentially point by point to cover the positive region, a slow process indeed.
In the presently invented method, the photo-curable binder may consist of such adhesive compositions as a base resin, a hardening or cross-linking agent, a photo-initiator, a photo-sensitizer, and possibly with additional catalyst and/or reaction accelerator. All of these compositions, if in a powder form, may be mixed together to form a complete binder adhesive mixture. This binder mixture is then attracted by the electro-photographic means to form into a binder image, which is transferred and deposited onto a powder layer. Alternatively, one or more compositions may be included as secondary ingredients in the primary body-building powder material to be dispensed one layer at a time by a powder feeder (powder-dispensing means) while the remaining composition(s) may constitute the binder powder image.
The powder inside a powder feeder may comprise a primary body-building material (fine particles), additives (physical or chemical property modifiers), and secondary ingredients (selected compositions of a binder adhesive). In this method, the primary body-building powder may be composed of one or more than one type of fine particles. These fine powder particles could be of any geometric shape, but preferably spherical. The particle sizes are preferably smaller than 100 xcexcm, further preferably smaller than 10 xcexcm, and most preferably smaller than 1 xcexcm. The size distribution is preferably uniform. The primary body-building powder may be selected from the following three basic types of powders:
Type A: fine particles of a primary body-building material only. In this type, only primary body-building materials in a fine particle form are included as the ingredients in the powder; no binder composition being included. All binder compositions are present as a binder powder to be formed into an image by the electro-photographic means. The primary body-building materials can be selected from polymers, ceramics, glass, metals and alloys, carbon, and combinations thereof. The polymers may be thermoplastic (e.g., polyvinyl chloride) or thermosetting (e.g., polyimide oligomer or prepolymer powder). The binder, including all selected compositions, will be deposited over the positive region of a complete layer and allowed to permeate through the gaps between fine particles in a layer of primary body-building powder. The binder (if an adhesive) in the positive region (corresponding to the desired cross-section) of a layer will be at least partially cured (chemically cross-linked or otherwise hardened) to bond together the primary body building particles. The binder (if containing a fusible material composition) will be heated to become a fluid which, once permeated through a layer, will be cooled to solidify. No binder will be deposited to the negative region and, hence, the fine particles in this region will remain loose and will stay as part of a support structure.
Type B: fine ceramic, metallic, glass, or polymeric particles (as primary body-building materials) each coated with a thin layer of coating comprising selected binder adhesive compositions. Once a layer of these coated solid particles is deposited, the remaining compositions of a binder adhesive are then deposited, melted, and allowed to permeate through the gaps between these primary body-building particles. These remaining compositions are then in contact or reacted with the selected binder compositions in the coating to make a complete binder adhesive. The binder adhesive, only existing in the positive region of a layer, is then at least partially cured by heat and/or UV light or any other energy means to bond together body-building particles, leaving the particles in the negative region loose and un-bonded.
Type C: a mixture of fine particles of primary body-building materials (e.g., a silicon dioxide powder) with at least one binder adhesive composition also in a fine powder form. The other remaining binder adhesive compositions are electro-photographically formed into a binder image, deposited onto a layer of Type C powder mixture, and allowed to flow around the fine particles and react with the at least one binder adhesive composition. The complete binder adhesive formulation in the positive region of this layer is then at least partially cured to provide inter-particle bonding for those primary body-building particles in the positive region. Again, the adhesive will not enter the negative region and the powder particles in this region will remain loose and physically separable.
In each powder type, additional ingredients may be added to impart desired physical and/or chemical properties to the object being built. These ingredients may contain a reinforcement composition selected from the group consisting of short fiber, whisker, and particulate reinforcements such as a spherical particle, ellipsoidal particle, flake, small platelet, small disc, etc. These ingredients may also contain, but not limited to, colorants, anti-oxidants, anti-corrosion agent, sintering agent, plasticizers, etc. Any of these ingredients, when intended to be used in each and every layer of the 3-D object, may preferably be included in the primary body-building powder to be dispensed by a traditional powder feeder. Those ingredients that are to be deposited only at selected spots of a layer or selected layers (but not all layers) of an object may be included as a part of a modifier powder. These ingredients will then be electro-photographically formed into a modifier powder image (toner) and transferred to a corresponding cross-section of a primary body-building powder, before or after the binder powder image is transferred. Alternatively, selected ingredients may be combined with a binder powder to form a composite binder-modifier powder image.
Many prior-art powder-dispensing means or feeders are available for feeding layers of powder materials, one layer at a time. The moving and dispensing operations of the powder-dispensing means and the operation of an electrophotographic powder deposition means are preferably conducted under the control of a computer. This can be accomplished by (1) first creating a computer-aided design of the 3-D object on a computer with the design containing information on both the geometry and material composition distribution of the object with the geometry including a plurality of data points defining the object, (2) generating programmed signals corresponding to each of the data points, collected into layer-wise data sets, in a predetermined sequence; (3) generating plural powder images (comprising a binder powder image and at least a modifier powder image) and transferring/depositing these binder/modifier powder images to corresponding areas of a layer of body-building powder material responsive to these programmed signals, (4) moving the powder-dispensing means and the work surface relative to each other (in Z-direction, e.g.) in response to these programmed signals. The signals for moving may be advantageously prescribed in accordance with the G-codes and M-codes that are commonly used in computer numerical control (CNC) machinery industry, but other motion control codes may also be used. The signals for forming a powder image may be created by any image formation means commonly used in an electrostatic printer or photo-copier.
In order to produce a multi-material 3-D object in which the material composition of the primary body-building powder can vary from layer to layer, the presently invented method may further comprise the steps of (1) creating a geometry of the 3-D object on a computer with the geometry including a plurality of layer-wise sets of data points defining the object; each of the data sets being coded with a selected material composition, (2) generating programmed signals corresponding to each of the data sets in a predetermined sequence; and (3) operating the powder-dispensing means in response to the programmed signals to dispense and deposit powders of selected body-building material compositions, with the material compositions varying possibly from layer to layer. In order to achieve a point-to-point variation in material composition or color, each data point may be coded with a material composition or color. Such a material distribution or color pattern can be physically achieved by using the color electrophotography steps to form and transfer multi-material or multi-color powder images to corresponding layers of a primary body-building powder. The virtual reality modeling language (VRML), which is capable of building the geometry of a 3-D object with rich material composition and/or color information, is particularly useful as a CAD tool in the practice of the present invention.
To further ensure the part accuracy and compensate for the potential variations in part dimensions (thickness, in particular), the present method may be executed under the assistance of dimension sensors. These sensors may be used to periodically measure the dimensions of the object being built while a computer is used to determine the thickness and outline of individual layers intermittently in accordance with a computer aided design representation of the object. The computing step includes operating the computer to calculate a first set of logical layers with specific thickness and outline for each layer and then periodically re-calculate another set of logical layers after periodically comparing the dimension data acquired by the sensor with the computer aided design representation in an adaptive manner.
The Apparatus
Another embodiment of this invention is a solid freeform fabrication apparatus for automated fabrication of a 3-D object. This apparatus includes:
(1) a work surface to support the object while being built;
(2) powder-dispensing means at a predetermined initial distance from the work surface; the dispensing means having an outlet directed to the work surface for feeding successive layers of powder onto the work surface, one layer at a time, with the powder including at least a primary body-building material;
(3) an electrophotographic powder deposition means at a distance from the work surface; the electrophotographic powder deposition means having an imaging surface directed to the work surface for feeding successive layers of binder/modifier powder images onto the corresponding layers of primary body-building materials, one layer at a time;
(4) energy means at a distance from the work surface for providing fusion, cooling, curing, and/or bonding energy to successive layers being built; and
(5) motion devices coupled to the work surface, electrophotographic powder deposition means, and powder-dispensing means for moving the electrophotographic and dispensing means with respect to the work surface so that the binder/modifier powder image plane is substantially parallel to a plane defined by first and second directions (X- and Y-directions) and in a third direction (Z-direction) orthogonal to the X-Y plane to dispense multiple layers of powder and then transferring binder/modifier powder images, one layer at a time, for forming the 3-D object. Preferably, the work surface is lowered by one layer thickness distance vertically in the Z-direction after one layer is built to get ready for receiving powders of the next layer.
In order to automate the object-fabricating process, the present apparatus is preferably equipped with a computer-aided design computer and supporting software programs operative to (a) create a three-dimensional geometry of the 3-D object, (b) convert this geometry into a plurality of data points defining geometry and material composition of the object, and (c) generate programmed signals corresponding to each of the data points in a predetermined sequence. The apparatus also includes a three-dimensional motion controller electronically linked to the computer and the motion devices. The electrophotographic powder deposition means is also preferably electronically connected to the computer, optionally through an electrophotography controller. The motion controller is operated to actuate the motion devices and the electrophotography controller is operated to activate the electrophotographic powder deposition means to generate a binder and/or modifier powder image, both being responsive to the programmed signals for the data points received from the computer.
The apparatus preferably includes dimension sensors that are electronically linked to the computer. The sensors periodically provide layer dimension data to the computer. In the mean time, the supporting software programs in the computer act to perform adaptive layer slicing to periodically create a new set of layer data, including the data points defining the object, in accordance with the layer dimension data acquired by the sensors means. New sets of programmed signals corresponding to each of the new data points are generated in a predetermined sequence.
Specifically, the motion devices are responsive to a CAD-defined data file which is created to represent the 3-D preform shape to be built. A geometry (drawing) of the object is first created in a CAD computer. The geometry is then sectioned into a desired number of layers with each layer being comprised of a plurality of data points. These layer data are then converted to form an image for attracting binder powder particles and also converted to machine control languages that can be used to drive the operation of the motion devices and powder-dispensing devices. These motion devices operate to provide relative rotational and translational motions of the powder-dispensing device and the electro-photographic powder deposition means with respect to the work surface. The motion devices further provide relative movements of the work surface in the Z-direction, each time by a predetermined thickness distance.
Advantages of the Invention
The process and apparatus of this invention have several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims which follow, its more prominent features will now be discussed briefly. After considering this brief discussion, and particularly after reading the section entitled xe2x80x9cDESCRIPTION OF THE PREFERRED EMBODIMENTSxe2x80x9d one will understand how the features of this invention offer its advantages, which include:
(1) The present invention provides a unique and novel method for producing a three-dimensional object on a layer-by-layer basis under the control of a computer. This method does not require the utilization of a pre-shaped mold or tooling.
(2) Most of the layer manufacturing methods, including powder-based techniques such as 3-D printing (3DP) and conventional selective laser sintering (SLS), are normally limited to the fabrication of an object in a point-by-point fashion and, hence, are very slow. In contrast, the presently invented method allows the fabrication of a part one complete layer at a time due to the full-field sized programmable, electrophotographic powder deposition device being capable of precisely forming a thin layer of binder powder corresponding to the positive region of a layer. Therefore, the presently invented method can be order-of-magnitude faster than the conventional SLS and 3DP.
(3) The presently invented method provides a computer-controlled process which places minimal constraint on the variety of materials that can be processed. In the present method, both the primary body-building powder material and the modifier powder may be selected from a broad array of materials including various organic (including polymers) and inorganic substances (including ceramic, metal, glass, and carbon based materials) and their mixtures. This is in sharp contrast to both Stereo Lithography (SLa) and Solid Ground Curing (SGC), which solely rely on ultraviolet (UV) light-curable polymers such as acrylate and epoxy resins as the primary body-building material. The photo-curable polymer in both SGC and SLa represents the vast majority of the material in the resulting 3-D structure and is the xe2x80x9cmatrixxe2x80x9d or xe2x80x9chostxe2x80x9d that accommodates any additive or reinforcement that might exist in the structure. The host basically provides the structural integrity of the 3-D object. The cured resin will not be removed or otherwise disintegrated. In the instant invention, the binder adhesive provides only a vehicle for tentatively holding together other otherwise loose powder particles. This binder or adhesive constitutes only a minority material phase of the resulting 3-D structure. In the cases of ceramic, glass, or metal powder particles, this cured adhesive will be burned off leading to the formation of a somewhat porous structure. This porous structure is then either sintered at a high temperature to produce a solid body or impregnated with another liquid material (e.g., metal melt) to form a composite or hybrid material object. This final structure will contain no low-temperature material such as the polymeric adhesive (only metal and/or ceramic, e.g.). Both metal and ceramic materials can be used in a much higher temperature environment.
xe2x80x83In terms of the variety of materials, the presently invented method also presents several advantages over the prior-art electrophotographic powder deposition based SFF techniques. For instance, these prior-art techniques are normally limited to the formation of thin, light weight powder images only and are not able to form a thicker layer of heavier powders such as ceramic and metallic particles due to the limited electrostatic attractive force between charges and solid powder particles. Further, it is normally very difficult to charge electrically conductive materials such as metals and, hence, the prior-art electro-photographic methods are not effective in building parts from metallic powders. In contrast, in the practice of our method, one is free to choose any light-weight, non-conductive binder powder composition to be electrophotographically formed and transferred to a layer of primary body-building powder. Individual layers of a heavier and/or conductive primary body-building powder such as a metal or ceramic material can be deposited by using other more simple and easy-to-perform powder-dispensing means (such as those successfully used in SLS and 3D-P), which are not limited by the relatively weak electrostatic attractive forces.
(4) The present method provides an adaptive layer-slicing approach and a thickness sensor to allow for in-process correction of any layer thickness variation. The present invention, therefore, offers a preferred method of layer manufacturing when part accuracy is a desirable feature.
(5) The method can be embodied using simple, inexpensive, and field-proven photo-copier mechanisms, so that the fabricator apparatus can be relatively small, light, inexpensive and easy to maintain. No high-power laser beam (to fuse and sinter a thicker layer of powder) is required.
(6) In the present method, a support structure naturally exists when a layer of body-building powder is fed. No additional tool is needed to build the support structure. This is in contrast to most of the prior-art layer-additive techniques that require a separate tool to build a support structure point by point, thereby slowing down the part-building process.